1. Field of the Invention
This invention relates to identification systems of moveable subjects and specifically to identification, friend or foe, IFF systems.
The basic requirements for an IFF system can be reduced to the following:
(a) it shall give a positive identification of friendly targets when required;
(b) it shall have acceptably low false alarm and false dismissal probabilities;
(c) it shall possess an acceptable measure of security such that the response cannot be readily counterfeited by the enemy; and
(d) it shall not endanger the respondent or be vulnerable to direction-finding receivers.
2. Description of the Prior Art
The conventional way to achieve an IFF function is for the moveable subject, such as an aircraft, to carry a transpoder that emits a coded return when a radar pulse is received by its receiver. This return transmission is normally on a different frequency from the radar, and is received at the radar site by a separate antenna and receiver. As the radiated return occurs simultaneously, or virtually so, with the reception of the radar signal, the range is related directly to the elapsed time, and the direction can be obtained by a suitably directive antenna. To obtain an adequate signal-to-noise ratio, a relatively high-power transponder transmitter is normally used of about 1 kw peak.
Such a system can be adapted directly for battlefield use, and meets all requirements stated above, except perhaps the last one. However, a separate antenna and receiver will have to be added to the radar and the moveable subject must carry a transponder, which raises problems of size, weight and power consumption. To minimize enemy detection, the transponder power output must be minimized and receiver antenna gain must be increased to compensate.